Analog signals contain frequency domain information and time domain information that may be of interest to persons analyzing the signals. For various reasons, analog signals are often sampled by a time-based sampling process prior to analyzing the signal. Data samples produced by the sampling process contain both time domain information and frequency domain information about the analog signal. The time domain information, such as amplitude, for example, can be analyzed by measuring the amplitude of the data samples with an appropriate measuring device, such as a voltmeter, for example. Likewise, the frequency domain information can be analyzed by an appropriate apparatus, such as a digital spectrum analyzer, for example.
In order to find time and frequency domain information for points between data samples, an interpolation technique must be used. The prior art offers numerous time-based methods of interpolation that provide varying degrees of accuracy between the interpolated values and the analog signal. Like the original data samples, time domain information contained in an interpolated value, such as amplitude, can then be measured by an appropriate device, such as a voltmeter.
Likewise, if frequency domain information is sought for points between data samples, the data samples must be interpolated to find these values. Unfortunately, a time-based sampling process that produces data samples of an analog signal also produces spectral images of the analog signal's frequency spectrum. As a result, the data samples contain frequency domain information that includes the analog signal frequency spectrum information plus image information. The image information corrupts the frequency spectrum information so that the frequency domain information contained in the data samples does not accurately replicate the frequency spectrum of the analog signal. Prior art interpolation techniques further corrupt the frequency information contained in the data samples. As a result, the frequency information contained in the interpolated values does not accurately represent the frequency information contained in the analog signal.
As can be appreciated from the foregoing discussion, there is a need to provide a method and apparatus for interpolating between data samples such that the interpolated values contain the same frequency information contained in the sampled analog signal, i.e., such that the frequency spectrum of the sampled analog signal is preserved. The present invention is directed to a method and apparatus for interpolating between data samples using a digital filter in a novel manner that achieves this result, i.e., produces interpolated values that preserve the frequency spectrum of an analog signal.